For example, let's say you have an idea for replacing online ads with a better monetization system for the benefit of the user. How do you pitch that at Google? A misstep here could literally destroy the company. It's insanity, akin to Exxon selling off their fossil fuel operation to try their hand at making solar panels.
Regulatory and PR risks are similarly grave. For example, Google couldn't have pulled off something like TikTok without all kinds of regulators jumping at their throats right away. They had to wait for ByteDance to clear the way and then launched their own "also-ran" clone. It's the same story with ChatGPT: Google had the tech but not the freedom to let it loose.
All of this is rational. You can get away with a lot more when you're a scrappy startup and don't have much to lose. When you're a multi-trillion-dollar company, the math ain't the same.